1. Field of the Invention
The present invention relates to a conductive paste composition and a solar cell element employing the same, in particular, to a conductive paste composition used for forming an electrode or wires on a silicon semiconductor substrate of a silicon wafer solar cell element, and a solar cell element made from the conductive paste composition.
2. Description of the Prior Art
Currently, as environmental problems such as energy shortage and green-house effect become increasingly severe, all countries are actively developing various possible alternative energy sources, and the greatest attention is paid to solar power generation. FIG. 1 is a schematic cross-sectional view of a solar cell element. Referring to FIG. 1, a surface of a p-type silicon semiconductor substrate 1 is fabricated into a pyramid-shaped rough surface so as to reduce the light reflection (the rough surface is not shown in FIG. 1 for simplicity of the drawing). Then, phosphorus or similar substance is thermally diffused to form an n-type impurity layer 2 having an opposite conductivity type on the side of a light-receiving surface of the p-type silicon semiconductor substrate 1, and form a p-n junction. Afterward, an anti-reflective layer 3 and an electrode 4 are formed on the n-type impurity layer 2. A silicon nitride film may be formed on the n-type impurity layer 2 by plasma chemical vapor deposition (CVD) as an anti-reflective layer 3. Then, a conductive silver paste containing a silver powder is coated on the anti-reflective layer 3 by screen printing, and then a drying and baking process and a sintering process are performed so as to form a front electrode 4. In the sintering process, the conductive paste used for forming the front electrode 4 may be sintered and penetrate into the n-type impurity layer 2. An aluminum back electrode layer 5 is formed on the backside of the p-type silicon semiconductor substrate 1 by printing, subsequently drying and baking, and then sintering at a high temperature a conductive aluminum paste containing an aluminum powder. In the sintering process, aluminum atoms diffuse into the p-type silicon semiconductor substrate 1, thereby forming an Al—Si alloy layer 6 between the aluminum back electrode layer 5 and the p-type silicon semiconductor substrate 1 and forming a p+ layer 7 doped with a high concentration of aluminum. The p+ layer 7 is generally referred to as a back surface field (BSF) layer and can prevent the recombination of electrons and holes and improve the energy conversion efficiency of solar cells. Moreover, in order to connect a plurality of solar cells in series to form a module, a conductive silver-aluminum paste may be printed on the aluminum back electrode layer 5 by screen printing, and then sintered to form a wire 8.
The back electrode may be formed by printing and drying the conductive silver-aluminum paste first, subsequently printing and drying the conductive aluminum paste, and then baking the two conductive pastes; and may also be formed by printing and drying the conductive aluminum paste first, subsequently printing the conductive silver-aluminum paste, and then drying and baking the two conductive pastes.
The conductive paste (silver paste, aluminum paste, and silver-aluminum paste) needed for the above process generally contain a conductive substance such as a silver powder or an aluminum powder, a glass powder, an organic vehicle, and other additives, where the organic vehicle includes a binder and a solvent. The organic vehicle functions to impart the conductive paste with a good compatibility and rheological property, for example, to impart the conductive paste with a suitable viscosity, good wettability and sintering property, and allow the powder in the conductive paste to be in a stable dispersion state. The binders commonly used in the art include polymethacrylates, ethyl cellulose, an alkyd resin, and the like. The solvents commonly used in the art include glycol ether-based organic solvents such as ethylene glycol monobutyl ether acetate and diethylene glycol monobutyl ether, or terpene-based solvents (for example, α-terpineol), and the like. The viscosity of the organic vehicle used in the art is adjusted with a solvent, so as to facilitate coating operations. However, as a large quantity of the solvent evaporates in the drying process after the printing, the environment will be contaminated, which is inconsistent with the concept of environmental protection. In addition, the drying process requires a long time, which becomes one of bottlenecks in increasing production capacity.
As conversion of sunlight into current is the most important characteristic of a solar cell, the photoelectric conversion efficiency is quite important. Among the conductive pastes, the silver paste in the front electrode has the greatest effect on the efficiency. Currently known methods for enhancing the photoelectric conversion efficiency include adjusting the composition of the glass powder or the silver powder in the silver paste, changing the particle size of the powder, and the like, as described in U.S. Pat. No. 4,235,644, U.S. Pat. No. 4,342,795, U.S. Pat. No. 5,661,041, and U.S. Pat. No. 7,176,152. The width and thickness of the printed wires or electrode, as well as wiring density and pattern, will affect the photoelectric conversion efficiency, depending on the selected conductive paste. The conductive paste will affect the width and thickness of the wires, and thus affect the light-receiving area and series impedance of silicon wafers. If the light-receiving area can be increased and the series impedance be reduced, the photoelectric conversion efficiency can be enhanced. However, as described above, currently known conductive pastes are solvent-based formulations containing a solvent, and when the silver electrode is fabricated on the light-receiving surface, as the wafer surface is roughened, the conductive silver paste will spread out during the printing due to the capillary action between the solvent and the roughened surface. Moreover, the flow of the paste due to gravity and the evaporation of the solvent during baking may cause the width of the wires to increase and the thickness of the wires to decrease, and meanwhile cause the light-shading area and the series impedance to increase. As a result, the photoelectric conversion efficiency is reduced.